diff --git a/apps/readest-app/package.json b/apps/readest-app/package.json
index 67710b44..575f8a28 100644
--- a/apps/readest-app/package.json
+++ b/apps/readest-app/package.json
@@ -182,6 +182,7 @@
     "react-dom": "19.2.5",
     "react-i18next": "^15.2.0",
     "react-icons": "^5.4.0",
+    "react-markdown": "^10.1.0",
     "react-responsive": "^10.0.0",
     "react-virtuoso": "^4.17.0",
     "react-window": "^1.8.11",
@@ -194,6 +195,7 @@
     "tauri-plugin-device-info-api": "^1.0.1",
     "tinycolor2": "^1.6.0",
     "uuid": "^14.0.0",
+    "virtua": "^0.49.1",
     "ws": "^8.18.3",
     "zod": "^4.0.8",
     "zustand": "5.0.10"
diff --git a/apps/readest-app/src/__tests__/reedy/reedyStore.test.ts b/apps/readest-app/src/__tests__/reedy/reedyStore.test.ts
new file mode 100644
index 00000000..acc28e39
--- /dev/null
+++ b/apps/readest-app/src/__tests__/reedy/reedyStore.test.ts
@@ -0,0 +1,203 @@
+import { describe, it, expect } from 'vitest';
+import {
+  applyEventToMessages,
+  type ReedyAssistantMessage,
+  type ReedyMessage,
+} from '@/services/reedy/store/reedyStore';
+import { makeToolError } from '@/services/reedy/runtime/errors';
+import type { ReedyEvent } from '@/services/reedy/runtime/events';
+
+function baseAssistant(id: string): ReedyAssistantMessage {
+  return { id, role: 'assistant', parts: [], createdAt: 0 };
+}
+
+describe('reedyStore — applyEventToMessages reducer', () => {
+  it('returns the input array unchanged when no matching assistant message exists', () => {
+    const before: ReedyMessage[] = [{ id: 'u1', role: 'user', text: 'hi', createdAt: 0 }];
+    const after = applyEventToMessages(before, 'missing', {
+      type: 'text_delta',
+      delta: 'x',
+    });
+    expect(after).toBe(before);
+  });
+
+  it('coalesces consecutive text_delta events into one text part', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', { type: 'text_delta', delta: 'Hello, ' });
+    msgs = applyEventToMessages(msgs, 'a1', { type: 'text_delta', delta: 'world.' });
+    const assistant = msgs[0] as ReedyAssistantMessage;
+    expect(assistant.parts).toHaveLength(1);
+    expect(assistant.parts[0]).toEqual({ type: 'text', text: 'Hello, world.' });
+  });
+
+  it('skips zero-length text_delta', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', { type: 'text_delta', delta: '' });
+    const assistant = msgs[0] as ReedyAssistantMessage;
+    expect(assistant.parts).toEqual([]);
+  });
+
+  it('appends a tool_call part in pending state', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_call',
+      id: 'tc1',
+      name: 'lookupPassage',
+      args: { query: 'alice' },
+      permission: 'read',
+    });
+    const part = (msgs[0] as ReedyAssistantMessage).parts[0]!;
+    expect(part.type).toBe('tool_call');
+    if (part.type === 'tool_call') {
+      expect(part.state).toBe('pending');
+      expect(part.name).toBe('lookupPassage');
+    }
+  });
+
+  it('tool_result ok transitions the matching tool_call to state=ok with result', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_call',
+      id: 'tc1',
+      name: 'find',
+      args: { q: 'x' },
+      permission: 'read',
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_result',
+      id: 'tc1',
+      name: 'find',
+      ok: true,
+      result: { hit: 'x' },
+      durationMs: 42,
+    });
+    const part = (msgs[0] as ReedyAssistantMessage).parts[0]!;
+    if (part.type === 'tool_call') {
+      expect(part.state).toBe('ok');
+      expect(part.result).toEqual({ hit: 'x' });
+      expect(part.durationMs).toBe(42);
+    }
+  });
+
+  it('tool_result error transitions the matching tool_call to state=error', () => {
+    const err = makeToolError('tool_runtime_error', 'find', 'boom');
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_call',
+      id: 'tc2',
+      name: 'find',
+      args: {},
+      permission: 'read',
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_result',
+      id: 'tc2',
+      name: 'find',
+      ok: false,
+      error: err,
+    });
+    const part = (msgs[0] as ReedyAssistantMessage).parts[0]!;
+    if (part.type === 'tool_call') {
+      expect(part.state).toBe('error');
+      expect(part.error?.kind).toBe('tool_runtime_error');
+    }
+  });
+
+  it('tool_result with no matching tool_call leaves parts untouched', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'tool_result',
+      id: 'orphan',
+      name: 'find',
+      ok: true,
+      result: {},
+      durationMs: 0,
+    });
+    expect((msgs[0] as ReedyAssistantMessage).parts).toEqual([]);
+  });
+
+  it('appends citation parts in arrival order', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'citation',
+      cfi: 'epubcfi(/6/2)',
+      sectionIndex: 0,
+      chapterTitle: 'Ch1',
+      snippet: 'first',
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'citation',
+      cfi: 'epubcfi(/6/4)',
+      sectionIndex: 1,
+      snippet: 'second',
+    });
+    const parts = (msgs[0] as ReedyAssistantMessage).parts;
+    expect(parts).toHaveLength(2);
+    expect(parts.every((p) => p.type === 'citation')).toBe(true);
+  });
+
+  it('error event appends an error part without dropping prior content', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', { type: 'text_delta', delta: 'partial' });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'error',
+      kind: 'model_error',
+      message: 'upstream 500',
+      retryable: true,
+    });
+    const parts = (msgs[0] as ReedyAssistantMessage).parts;
+    expect(parts).toHaveLength(2);
+    expect(parts[0]!.type).toBe('text');
+    expect(parts[1]!.type).toBe('error');
+  });
+
+  it('abort event appends an abort part with the partial flag', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', { type: 'abort', partial: true });
+    const part = (msgs[0] as ReedyAssistantMessage).parts[0]!;
+    expect(part).toEqual({ type: 'abort', partial: true });
+  });
+
+  it('done event captures finishReason + usage on the assistant message', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    const ev: ReedyEvent = {
+      type: 'done',
+      output: {
+        sessionId: 's1',
+        assistantMessageId: 'a1',
+        finishReason: 'stop',
+        usage: { promptTokens: 100, completionTokens: 50 },
+      },
+    };
+    msgs = applyEventToMessages(msgs, 'a1', ev);
+    const assistant = msgs[0] as ReedyAssistantMessage;
+    expect(assistant.finishReason).toBe('stop');
+    expect(assistant.usage).toEqual({ promptTokens: 100, completionTokens: 50 });
+  });
+
+  it('non-structural events (turn_start, step_finish, usage, memory_write) leave parts unchanged', () => {
+    let msgs: ReedyMessage[] = [baseAssistant('a1')];
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'turn_start',
+      sessionId: 's1',
+      assistantMessageId: 'a1',
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'step_finish',
+      step: 0,
+      reason: 'stop',
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'usage',
+      promptTokens: 10,
+      completionTokens: 5,
+    });
+    msgs = applyEventToMessages(msgs, 'a1', {
+      type: 'memory_write',
+      scope: 'book',
+      key: 'k',
+      summary: 's',
+    });
+    expect((msgs[0] as ReedyAssistantMessage).parts).toEqual([]);
+  });
+});
diff --git a/apps/readest-app/src/app/reader/components/notebook/AIAssistant.tsx b/apps/readest-app/src/app/reader/components/notebook/AIAssistant.tsx
index ab32da77..f03f2e54 100644
--- a/apps/readest-app/src/app/reader/components/notebook/AIAssistant.tsx
+++ b/apps/readest-app/src/app/reader/components/notebook/AIAssistant.tsx
@@ -28,6 +28,8 @@ import type { RetrievedChunk } from '@/services/reedy/retrieval/BookRetriever';
 import { useEnv } from '@/context/EnvContext';
 import { isTauriAppPlatform } from '@/services/environment';
 import type { AppService } from '@/types/system';
+import { ReedyAssistant } from '@/services/reedy/ui/ReedyAssistant';
+import type { ReadingContextSnapshot } from '@/services/reedy/tools/builtins/types';
 
 import { Button } from '@/components/ui/button';
 import { Loader2Icon, BookOpenIcon } from 'lucide-react';
@@ -276,7 +278,34 @@ const ThreadWrapper = ({
   );
 };
 
+/**
+ * Phase 4.3 router. Switches between the legacy / Reedy-MVP path
+ * (LegacyAIAssistant) and the Phase 4 agent-runtime path
+ * (ReedyAgentAssistantBridge) based on aiSettings.reedy.runtime.
+ *
+ * The split is at component boundary rather than inside one component
+ * so hooks always run in stable order on whichever path is rendered.
+ */
 const AIAssistant = ({ bookKey }: AIAssistantProps) => {
+  const { appService } = useEnv();
+  const { settings } = useSettingsStore();
+  const { getBookData } = useBookDataStore();
+  const bookData = getBookData(bookKey);
+
+  const reedyRuntime = settings?.aiSettings?.reedy?.runtime ?? 'mvp';
+  const useAgentRuntime =
+    settings?.aiSettings?.enabled === true &&
+    settings?.aiSettings?.reedy?.enabled === true &&
+    reedyRuntime === 'agent' &&
+    !!appService &&
+    isTauriAppPlatform() &&
+    !!bookData?.bookDoc;
+
+  if (useAgentRuntime) return <ReedyAgentAssistantBridge bookKey={bookKey} />;
+  return <LegacyAIAssistant bookKey={bookKey} />;
+};
+
+const LegacyAIAssistant = ({ bookKey }: AIAssistantProps) => {
   const _ = useTranslation();
   const { appService } = useEnv();
   const { settings } = useSettingsStore();
@@ -436,4 +465,55 @@ const AIAssistant = ({ bookKey }: AIAssistantProps) => {
   );
 };
 
+/**
+ * Bridge from the notebook AI tab into the Phase 4 ReedyAssistant.
+ *
+ * Kept separate from AIAssistant so legacy props/state don't leak in
+ * and we don't pay the cost of constructing the agent runtime when the
+ * user is on the MVP path. The flag check in AIAssistant guarantees this
+ * only renders when aiSettings.reedy.runtime === 'agent'.
+ */
+const ReedyAgentAssistantBridge = ({ bookKey }: AIAssistantProps) => {
+  const { appService } = useEnv();
+  const { settings } = useSettingsStore();
+  const { getBookData } = useBookDataStore();
+  const { getProgress, getView } = useReaderStore();
+  const bookData = getBookData(bookKey);
+  const progress = getProgress(bookKey);
+
+  const bookHash = bookKey.split('-')[0] || '';
+  const aiSettings = settings?.aiSettings;
+
+  const readingContext = useMemo<ReadingContextSnapshot>(
+    () => ({
+      cfi: progress?.location ?? null,
+      sectionIndex: progress?.section?.current ?? 0,
+      chapterTitle: progress?.sectionLabel ?? null,
+      pageNumber: progress?.pageinfo?.current ?? 0,
+    }),
+    [progress],
+  );
+
+  const handleNavigate = useCallback(
+    (cfi: string) => {
+      getView(bookKey)?.goTo(cfi);
+    },
+    [bookKey, getView],
+  );
+
+  if (!aiSettings || !appService || !bookData?.bookDoc) return null;
+
+  return (
+    <ReedyAssistant
+      appService={appService as AppService}
+      bookDoc={bookData.bookDoc}
+      bookHash={bookHash}
+      bookKey={bookKey}
+      aiSettings={aiSettings}
+      readingContext={readingContext}
+      onNavigateToCfi={handleNavigate}
+    />
+  );
+};
+
 export default AIAssistant;
diff --git a/apps/readest-app/src/components/settings/AIPanel.tsx b/apps/readest-app/src/components/settings/AIPanel.tsx
index d878c4f6..66fa7abe 100644
--- a/apps/readest-app/src/components/settings/AIPanel.tsx
+++ b/apps/readest-app/src/components/settings/AIPanel.tsx
@@ -76,6 +76,9 @@ const AIPanel: React.FC = () => {
 
   const [enabled, setEnabled] = useState(aiSettings.enabled);
   const [reedyEnabled, setReedyEnabled] = useState(aiSettings.reedy?.enabled ?? false);
+  const [reedyAgentRuntime, setReedyAgentRuntime] = useState(
+    (aiSettings.reedy?.runtime ?? 'mvp') === 'agent',
+  );
   const [provider, setProvider] = useState<AIProviderName>(aiSettings.provider);
   const [ollamaUrl, setOllamaUrl] = useState(aiSettings.ollamaBaseUrl);
   const [ollamaModel, setOllamaModel] = useState(aiSettings.ollamaModel);
@@ -760,7 +763,26 @@ const AIPanel: React.FC = () => {
           onChange={() => {
             const next = !reedyEnabled;
             setReedyEnabled(next);
-            saveAiSetting('reedy', { enabled: next });
+            saveAiSetting('reedy', {
+              enabled: next,
+              runtime: reedyAgentRuntime ? 'agent' : 'mvp',
+            });
+          }}
+        />
+        <SettingsSwitchRow
+          label={_('Use agent runtime (experimental)')}
+          description={_(
+            'When on, the notebook AI tab uses the new custom agent runtime + thread UI. When off, the Phase 1B MVP path (legacy assistant-ui Thread) is used.',
+          )}
+          checked={reedyAgentRuntime}
+          disabled={!enabled || !reedyEnabled || !isTauriAppPlatform()}
+          onChange={() => {
+            const next = !reedyAgentRuntime;
+            setReedyAgentRuntime(next);
+            saveAiSetting('reedy', {
+              enabled: reedyEnabled,
+              runtime: next ? 'agent' : 'mvp',
+            });
           }}
         />
         <div className='flex min-h-14 items-center justify-between gap-3 ps-4 pe-4'>
diff --git a/apps/readest-app/src/services/ai/types.ts b/apps/readest-app/src/services/ai/types.ts
index 85398b31..5e5c8880 100644
--- a/apps/readest-app/src/services/ai/types.ts
+++ b/apps/readest-app/src/services/ai/types.ts
@@ -45,6 +45,14 @@ export interface AISettings {
    */
   reedy?: {
     enabled: boolean;
+    /**
+     * 'mvp' (default) keeps the Phase 1B path: lookupPassage tool wired
+     * through @assistant-ui/react's adapter. 'agent' switches the
+     * notebook AI tab to the Phase 4 ReedyAssistant (custom AgentRuntime
+     * + thread UI). Requires `reedy.enabled && isTauri() &&
+     * runtime === 'agent'` to engage.
+     */
+    runtime?: 'mvp' | 'agent';
   };
 }
 
diff --git a/apps/readest-app/src/services/reedy/store/reedyStore.ts b/apps/readest-app/src/services/reedy/store/reedyStore.ts
new file mode 100644
index 00000000..e7362777
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/store/reedyStore.ts
@@ -0,0 +1,239 @@
+import { create } from 'zustand';
+import type { ReedyEvent, ReedyTurnOutput } from '../runtime/events';
+import type { ReedyToolError } from '../runtime/errors';
+
+/**
+ * Reedy chat store (Phase 4.1).
+ *
+ * Holds the per-session message log the agent UI renders. Distinct from
+ * the MVP `useAIChatStore` so the two paths never share state — switching
+ * `aiSettings.reedy.runtime` mid-session shouldn't corrupt either log.
+ *
+ * Messages are stored as ordered arrays of structured parts produced by
+ * the AgentRuntime's ReedyEvent stream. The store has no opinions about
+ * persistence — sessions today live in-memory only; Phase 6+ wires up
+ * disk storage if the agent path graduates beyond the measurement plan.
+ */
+
+export type ReedyMessagePart =
+  | { type: 'text'; text: string }
+  | {
+      type: 'tool_call';
+      id: string;
+      name: string;
+      args: unknown;
+      permission: 'read' | 'navigate' | 'write';
+      state: 'pending' | 'ok' | 'error';
+      result?: unknown;
+      error?: ReedyToolError;
+      durationMs?: number;
+    }
+  | {
+      type: 'citation';
+      cfi: string;
+      sectionIndex: number;
+      chapterTitle?: string;
+      snippet: string;
+    }
+  | { type: 'error'; message: string; kind: string }
+  | { type: 'abort'; partial: boolean };
+
+export interface ReedyUserMessage {
+  id: string;
+  role: 'user';
+  text: string;
+  createdAt: number;
+}
+
+export interface ReedyAssistantMessage {
+  id: string;
+  role: 'assistant';
+  parts: ReedyMessagePart[];
+  createdAt: number;
+  /** Set once the runtime emits `done`. */
+  finishReason?: ReedyTurnOutput['finishReason'];
+  usage?: ReedyTurnOutput['usage'];
+}
+
+export type ReedyMessage = ReedyUserMessage | ReedyAssistantMessage;
+
+export interface ReedyStoreState {
+  messages: ReedyMessage[];
+  /** True while a turn is being processed by the runtime. */
+  isRunning: boolean;
+  /** The active turn's assistantMessageId so we know which message to mutate on each event. */
+  activeAssistantMessageId: string | null;
+  /** AbortController for the active turn; the Composer's Stop button calls .abort(). */
+  abortController: AbortController | null;
+
+  // Mutations
+  startUserTurn: (text: string) => void;
+  startAssistantTurn: (assistantMessageId: string, controller: AbortController) => void;
+  applyEvent: (event: ReedyEvent) => void;
+  finishTurn: () => void;
+  reset: () => void;
+}
+
+export const useReedyStore = create<ReedyStoreState>((set, get) => ({
+  messages: [],
+  isRunning: false,
+  activeAssistantMessageId: null,
+  abortController: null,
+
+  startUserTurn(text) {
+    const msg: ReedyUserMessage = {
+      id: randomId('user'),
+      role: 'user',
+      text,
+      createdAt: Date.now(),
+    };
+    set((s) => ({ messages: [...s.messages, msg], isRunning: true }));
+  },
+
+  startAssistantTurn(assistantMessageId, controller) {
+    const msg: ReedyAssistantMessage = {
+      id: assistantMessageId,
+      role: 'assistant',
+      parts: [],
+      createdAt: Date.now(),
+    };
+    set((s) => ({
+      messages: [...s.messages, msg],
+      activeAssistantMessageId: assistantMessageId,
+      abortController: controller,
+    }));
+  },
+
+  applyEvent(event) {
+    const id = get().activeAssistantMessageId;
+    if (!id) return;
+    set((s) => ({ messages: applyEventToMessages(s.messages, id, event) }));
+  },
+
+  finishTurn() {
+    set({ isRunning: false, activeAssistantMessageId: null, abortController: null });
+  },
+
+  reset() {
+    set({
+      messages: [],
+      isRunning: false,
+      activeAssistantMessageId: null,
+      abortController: null,
+    });
+  },
+}));
+
+// ---------------------------------------------------------------------------
+// pure helpers — exported so tests can target the reducer without React.
+// ---------------------------------------------------------------------------
+
+export function applyEventToMessages(
+  messages: ReedyMessage[],
+  assistantMessageId: string,
+  event: ReedyEvent,
+): ReedyMessage[] {
+  const idx = messages.findIndex((m) => m.id === assistantMessageId && m.role === 'assistant');
+  if (idx === -1) return messages;
+  const current = messages[idx] as ReedyAssistantMessage;
+  const next = applyEventToAssistant(current, event);
+  if (next === current) return messages;
+  const out = messages.slice();
+  out[idx] = next;
+  return out;
+}
+
+function applyEventToAssistant(
+  msg: ReedyAssistantMessage,
+  event: ReedyEvent,
+): ReedyAssistantMessage {
+  switch (event.type) {
+    case 'text_delta':
+      return appendOrExtendText(msg, event.delta);
+    case 'tool_call':
+      return {
+        ...msg,
+        parts: [
+          ...msg.parts,
+          {
+            type: 'tool_call',
+            id: event.id,
+            name: event.name,
+            args: event.args,
+            permission: event.permission,
+            state: 'pending',
+          },
+        ],
+      };
+    case 'tool_result':
+      return {
+        ...msg,
+        parts: msg.parts.map((p) => {
+          if (p.type !== 'tool_call' || p.id !== event.id) return p;
+          if (event.ok) {
+            return {
+              ...p,
+              state: 'ok',
+              result: event.result,
+              durationMs: event.durationMs,
+            };
+          }
+          return { ...p, state: 'error', error: event.error };
+        }),
+      };
+    case 'citation':
+      return {
+        ...msg,
+        parts: [
+          ...msg.parts,
+          {
+            type: 'citation',
+            cfi: event.cfi,
+            sectionIndex: event.sectionIndex,
+            chapterTitle: event.chapterTitle,
+            snippet: event.snippet,
+          },
+        ],
+      };
+    case 'error':
+      return {
+        ...msg,
+        parts: [...msg.parts, { type: 'error', message: event.message, kind: event.kind }],
+      };
+    case 'abort':
+      return {
+        ...msg,
+        parts: [...msg.parts, { type: 'abort', partial: event.partial }],
+      };
+    case 'done':
+      return { ...msg, finishReason: event.output.finishReason, usage: event.output.usage };
+    case 'turn_start':
+    case 'step_finish':
+    case 'usage':
+    case 'memory_write':
+      // No structural-part mutation today. Phase 4 follow-up may want a
+      // separate ReasoningPart or MemoryNotePart; the store would gain
+      // a new branch then.
+      return msg;
+    default:
+      return msg;
+  }
+}
+
+function appendOrExtendText(msg: ReedyAssistantMessage, delta: string): ReedyAssistantMessage {
+  if (delta.length === 0) return msg;
+  const last = msg.parts.at(-1);
+  // Coalesce consecutive text deltas into one text part so the UI
+  // re-renders the same React node instead of inflating the parts array.
+  if (last && last.type === 'text') {
+    const updated = { ...last, text: last.text + delta };
+    return { ...msg, parts: [...msg.parts.slice(0, -1), updated] };
+  }
+  return { ...msg, parts: [...msg.parts, { type: 'text', text: delta }] };
+}
+
+function randomId(prefix: string): string {
+  if (typeof crypto !== 'undefined' && 'randomUUID' in crypto)
+    return `${prefix}-${crypto.randomUUID()}`;
+  return `${prefix}-${Date.now()}-${Math.random().toString(36).slice(2, 10)}`;
+}
diff --git a/apps/readest-app/src/services/reedy/ui/AgentThread.tsx b/apps/readest-app/src/services/reedy/ui/AgentThread.tsx
new file mode 100644
index 00000000..32a10a66
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/AgentThread.tsx
@@ -0,0 +1,65 @@
+'use client';
+
+import { useEffect, useRef, useState } from 'react';
+import { VList, type VListHandle } from 'virtua';
+import type { ReedyMessage } from '../store/reedyStore';
+import { MessageCard } from './MessageCard';
+
+/**
+ * Virtua-virtualized thread scroller (Phase 4.2.b).
+ *
+ * Owns auto-scroll engagement: pinned to the bottom on initial mount
+ * and after every store update while the user hasn't actively scrolled
+ * away. Pointer/wheel/touch interactions disengage; scrolling back to
+ * the bottom (or sending a new message) re-engages.
+ */
+export function AgentThread({
+  messages,
+  isRunning,
+  onSourceClick,
+  emptyState,
+}: {
+  messages: ReedyMessage[];
+  isRunning: boolean;
+  onSourceClick?: (cfi: string) => void;
+  emptyState?: React.ReactNode;
+}) {
+  const ref = useRef<VListHandle>(null);
+  const [autoScroll, setAutoScroll] = useState(true);
+
+  // Pin to the bottom whenever the message count grows (new turn, new
+  // assistant message, new structural part) AND we're in autoScroll mode.
+  useEffect(() => {
+    if (!autoScroll || messages.length === 0) return;
+    const lastIndex = messages.length - 1;
+    requestAnimationFrame(() => {
+      ref.current?.scrollToIndex(lastIndex, { align: 'end' });
+    });
+  }, [messages.length, autoScroll, messages]);
+
+  // Disengage auto-scroll when the user actively scrolls; re-engage when
+  // they reach the bottom again.
+  const handleScroll = (offset: number): void => {
+    const handle = ref.current;
+    if (!handle) return;
+    const total = handle.scrollSize;
+    const view = handle.viewportSize;
+    const atBottom = total - (offset + view) < 24;
+    setAutoScroll(atBottom);
+  };
+
+  if (messages.length === 0 && emptyState) {
+    return <div className='flex h-full items-center justify-center'>{emptyState}</div>;
+  }
+
+  return (
+    <VList ref={ref} className='reedy-agent-thread h-full w-full' onScroll={handleScroll}>
+      {messages.map((m) => (
+        <MessageCard key={m.id} message={m} onSourceClick={onSourceClick} />
+      ))}
+      {isRunning && messages.length > 0 && (
+        <div className='text-base-content/40 mb-4 px-3 text-[11px] italic'>Reedy is thinking…</div>
+      )}
+    </VList>
+  );
+}
diff --git a/apps/readest-app/src/services/reedy/ui/Composer.tsx b/apps/readest-app/src/services/reedy/ui/Composer.tsx
new file mode 100644
index 00000000..5fe15bb6
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/Composer.tsx
@@ -0,0 +1,134 @@
+'use client';
+
+import { useCallback, useState, type KeyboardEvent } from 'react';
+import { Send, Square, WandSparkles } from 'lucide-react';
+
+/**
+ * Minimal Skill shape the composer's chip row renders. The full Skill
+ * type (Phase 5 — separate PR) is a strict superset; the composer only
+ * touches these fields, so we declare locally to avoid a cross-branch
+ * dependency.
+ */
+export interface ComposerSkill {
+  id: string;
+  name: string;
+  description: string;
+}
+type Skill = ComposerSkill;
+
+/**
+ * Multi-line input + send/abort button + skill chip row (Phase 4.2.h).
+ *
+ * Keyboard:
+ *   - Cmd/Ctrl + Enter → send
+ *   - Esc              → abort if a turn is running, otherwise blur
+ *   - Enter alone      → newline (per the plan's UX)
+ */
+export function Composer({
+  isRunning,
+  onSend,
+  onAbort,
+  disabled,
+  skills,
+  activeSkillId,
+  onSkillSelect,
+}: {
+  isRunning: boolean;
+  onSend: (text: string) => void;
+  onAbort: () => void;
+  disabled?: boolean;
+  skills?: Skill[];
+  activeSkillId?: string | null;
+  onSkillSelect?: (id: string | null) => void;
+}) {
+  const [text, setText] = useState('');
+
+  const send = useCallback(() => {
+    const trimmed = text.trim();
+    if (trimmed.length === 0) return;
+    onSend(trimmed);
+    setText('');
+  }, [text, onSend]);
+
+  const handleKeyDown = (e: KeyboardEvent<HTMLTextAreaElement>): void => {
+    if (e.key === 'Enter' && (e.metaKey || e.ctrlKey)) {
+      e.preventDefault();
+      if (!disabled && !isRunning) send();
+    } else if (e.key === 'Escape') {
+      if (isRunning) {
+        e.preventDefault();
+        onAbort();
+      }
+    }
+  };
+
+  return (
+    <div className='reedy-agent-composer border-base-content/10 bg-base-100 flex flex-col gap-2 border-t p-2'>
+      {skills && skills.length > 0 && (
+        <div className='flex flex-wrap items-center gap-1'>
+          <span className='text-base-content/40 me-1 text-[10px] uppercase'>Skill</span>
+          <button
+            type='button'
+            className={chipClass(activeSkillId == null)}
+            onClick={() => onSkillSelect?.(null)}
+          >
+            None
+          </button>
+          {skills.map((s) => (
+            <button
+              key={s.id}
+              type='button'
+              className={chipClass(activeSkillId === s.id)}
+              onClick={() => onSkillSelect?.(s.id)}
+              title={s.description}
+            >
+              <WandSparkles className='size-3' />
+              {s.name}
+            </button>
+          ))}
+        </div>
+      )}
+
+      <div className='border-base-content/10 bg-base-200/40 eink-bordered flex items-end gap-1 rounded-md border px-2 py-1.5'>
+        <textarea
+          className='text-base-content placeholder:text-base-content/40 max-h-40 min-h-[1.75rem] flex-1 resize-none bg-transparent text-sm outline-none'
+          rows={1}
+          placeholder='Ask Reedy about this book…'
+          value={text}
+          disabled={disabled}
+          onChange={(e) => setText(e.target.value)}
+          onKeyDown={handleKeyDown}
+        />
+        {isRunning ? (
+          <button
+            type='button'
+            className='btn btn-primary btn-sm size-7 min-h-0 rounded-full p-0'
+            onClick={onAbort}
+            title='Stop (Esc)'
+            aria-label='Stop'
+          >
+            <Square className='size-3' />
+          </button>
+        ) : (
+          <button
+            type='button'
+            className='btn btn-primary btn-sm size-7 min-h-0 rounded-full p-0 disabled:opacity-40'
+            onClick={send}
+            disabled={disabled || text.trim().length === 0}
+            title='Send (⌘/Ctrl + Enter)'
+            aria-label='Send'
+          >
+            <Send className='size-3' />
+          </button>
+        )}
+      </div>
+    </div>
+  );
+}
+
+function chipClass(active: boolean): string {
+  return [
+    'border-base-content/10 flex items-center gap-1 rounded-full border px-2 py-0.5 text-[11px] transition-colors',
+    active ? 'bg-primary text-primary-content border-primary' : 'bg-base-100 hover:bg-base-200',
+  ].join(' ');
+}
diff --git a/apps/readest-app/src/services/reedy/ui/IndexingStatus.tsx b/apps/readest-app/src/services/reedy/ui/IndexingStatus.tsx
new file mode 100644
index 00000000..83c21d8d
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/IndexingStatus.tsx
@@ -0,0 +1,114 @@
+'use client';
+
+import { BookOpenIcon, Loader2Icon, RotateCw, AlertTriangle } from 'lucide-react';
+
+export type IndexingPhase = 'idle' | 'indexing' | 'indexed' | 'failed' | 'empty';
+
+export interface IndexingStatusProps {
+  status: IndexingPhase;
+  /** Phase-specific progress: 0–100 for indexing, omitted otherwise. */
+  progressPercent?: number;
+  /** Number of chunks processed vs total — shown inline while indexing. */
+  chunkProgress?: { current: number; total: number };
+  /** Error text shown when status='failed'. */
+  errorMessage?: string;
+  onIndex?: () => void;
+  onReindex?: () => void;
+  onCancel?: () => void;
+}
+
+/**
+ * Top-of-thread status bar (Phase 4.2.h). Surfaces the book's indexing
+ * lifecycle and offers the corresponding action button.
+ */
+export function IndexingStatus({
+  status,
+  progressPercent,
+  chunkProgress,
+  errorMessage,
+  onIndex,
+  onReindex,
+  onCancel,
+}: IndexingStatusProps) {
+  if (status === 'idle') {
+    return (
+      <div className='flex h-full flex-col items-center justify-center gap-3 p-4 text-center'>
+        <div className='bg-primary/10 rounded-full p-3'>
+          <BookOpenIcon className='text-primary size-6' />
+        </div>
+        <div>
+          <h3 className='text-base-content mb-0.5 text-sm font-medium'>Index this book</h3>
+          <p className='text-base-content/60 text-xs'>Enable agent search + chat for this book.</p>
+        </div>
+        <button className='btn btn-primary btn-sm h-8 text-xs' onClick={onIndex}>
+          <BookOpenIcon className='me-1.5 size-3.5' />
+          Start indexing
+        </button>
+      </div>
+    );
+  }
+
+  if (status === 'indexing') {
+    const pct = progressPercent ?? 0;
+    return (
+      <div className='flex h-full flex-col items-center justify-center gap-3 p-4 text-center'>
+        <Loader2Icon className='text-primary size-6 animate-spin' />
+        <div>
+          <p className='text-base-content mb-1 text-sm font-medium'>Indexing book…</p>
+          <p className='text-base-content/60 text-xs'>
+            {chunkProgress
+              ? `${chunkProgress.current} / ${chunkProgress.total} chunks`
+              : 'Preparing…'}
+          </p>
+        </div>
+        <div className='bg-base-200 h-1.5 w-32 overflow-hidden rounded-full'>
+          <div
+            className='bg-primary h-full transition-all duration-300'
+            style={{ width: `${pct}%` }}
+          />
+        </div>
+        {onCancel && (
+          <button className='btn btn-ghost btn-xs text-xs' onClick={onCancel}>
+            Cancel
+          </button>
+        )}
+      </div>
+    );
+  }
+
+  if (status === 'failed') {
+    return (
+      <div className='flex h-full flex-col items-center justify-center gap-3 p-4 text-center'>
+        <div className='bg-warning/10 rounded-full p-3'>
+          <AlertTriangle className='text-warning size-6' />
+        </div>
+        <div>
+          <h3 className='text-base-content mb-0.5 text-sm font-medium'>Indexing failed</h3>
+          <p className='text-base-content/60 text-xs'>{errorMessage ?? 'Unknown error.'}</p>
+        </div>
+        <button className='btn btn-outline btn-sm h-8 text-xs' onClick={onReindex}>
+          <RotateCw className='me-1.5 size-3.5' />
+          Retry indexing
+        </button>
+      </div>
+    );
+  }
+
+  if (status === 'empty') {
+    return (
+      <div className='flex h-full flex-col items-center justify-center gap-3 p-4 text-center'>
+        <BookOpenIcon className='text-base-content/40 size-6' />
+        <div>
+          <h3 className='text-base-content mb-0.5 text-sm font-medium'>No extractable text</h3>
+          <p className='text-base-content/60 text-xs'>
+            This book contains no extractable text (likely an image-only PDF or scanned book). Reedy
+            can't answer questions about its content.
+          </p>
+        </div>
+      </div>
+    );
+  }
+
+  // status === 'indexed' — nothing visible; the thread renders.
+  return null;
+}
diff --git a/apps/readest-app/src/services/reedy/ui/MessageCard.tsx b/apps/readest-app/src/services/reedy/ui/MessageCard.tsx
new file mode 100644
index 00000000..c57bbf9a
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/MessageCard.tsx
@@ -0,0 +1,87 @@
+'use client';
+
+import { memo } from 'react';
+import type { ReedyMessage, ReedyMessagePart } from '../store/reedyStore';
+import { AssistantTextPart, UserTextPart } from './parts/TextPart';
+import { ToolCallPart } from './parts/ToolCallPart';
+import { CitationPart } from './parts/CitationPart';
+import { AbortPart, ErrorPart } from './parts/StatusParts';
+
+/**
+ * One row in the agent thread. User messages render as a single text
+ * bubble; assistant messages dispatch each structural part to its
+ * dedicated renderer (text, tool_call, citation, error, abort).
+ *
+ * Memoized on the message reference — the store reducer creates a new
+ * object only for messages that mutate this tick, so unchanged rows
+ * skip the entire React subtree.
+ */
+export const MessageCard = memo(function MessageCard({
+  message,
+  onSourceClick,
+}: {
+  message: ReedyMessage;
+  onSourceClick?: (cfi: string) => void;
+}) {
+  if (message.role === 'user') {
+    return (
+      <div className='animate-in fade-in mx-auto mb-3 flex w-full justify-end duration-200'>
+        <div className='bg-base-200/60 text-base-content max-w-[85%] rounded-lg px-3 py-2 text-sm'>
+          <UserTextPart text={message.text} />
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div className='animate-in fade-in mb-4 flex w-full duration-200'>
+      <div className='flex w-full min-w-0 flex-col gap-1'>
+        {message.parts.map((part, i) => (
+          <PartDispatcher key={partKey(part, i)} part={part} onSourceClick={onSourceClick} />
+        ))}
+        {message.finishReason === 'error' && message.parts.every((p) => p.type !== 'error') && (
+          <ErrorPart part={{ type: 'error', kind: 'unknown', message: 'Turn ended in error.' }} />
+        )}
+      </div>
+    </div>
+  );
+});
+
+function PartDispatcher({
+  part,
+  onSourceClick,
+}: {
+  part: ReedyMessagePart;
+  onSourceClick?: (cfi: string) => void;
+}) {
+  switch (part.type) {
+    case 'text':
+      return <AssistantTextPart text={part.text} />;
+    case 'tool_call':
+      return <ToolCallPart part={part} />;
+    case 'citation':
+      return <CitationPart part={part} onClick={onSourceClick} />;
+    case 'error':
+      return <ErrorPart part={part} />;
+    case 'abort':
+      return <AbortPart part={part} />;
+    default:
+      return null;
+  }
+}
+
+function partKey(part: ReedyMessagePart, index: number): string {
+  switch (part.type) {
+    case 'tool_call':
+      return `tool:${part.id}`;
+    case 'citation':
+      return `citation:${part.cfi}:${index}`;
+    case 'text':
+      // Coalesced text part — index is stable once added.
+      return `text:${index}`;
+    case 'error':
+      return `error:${index}`;
+    case 'abort':
+      return `abort:${index}`;
+  }
+}
diff --git a/apps/readest-app/src/services/reedy/ui/ReedyAssistant.tsx b/apps/readest-app/src/services/reedy/ui/ReedyAssistant.tsx
new file mode 100644
index 00000000..6ee973b9
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/ReedyAssistant.tsx
@@ -0,0 +1,247 @@
+'use client';
+
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import type { AppService } from '@/types/system';
+import type { BookDoc } from '@/libs/document';
+import type { AISettings } from '@/services/ai/types';
+import { AgentRuntime } from '../runtime/AgentRuntime';
+import { BookIndexer } from '../retrieval/BookIndexer';
+import { BookRetriever } from '../retrieval/BookRetriever';
+import { ReedyDb } from '../db/ReedyDb';
+import { createReedyModels } from '../models/registry';
+import { ToolRegistry } from '../tools/ToolRegistry';
+import {
+  createAddCitationTool,
+  createGetReadingContextTool,
+  createGetSelectionTool,
+  createLookupPassageTool,
+  type ReadingContextSnapshot,
+} from '../tools/builtins';
+import {
+  DEFAULT_POLICY,
+  createPolicyLayer,
+  createReadingLayer,
+  createSkillLayer,
+  createToolCatalogLayer,
+} from '../context';
+import { useReedyStore } from '../store/reedyStore';
+import { useReedyTurn } from './useReedyTurn';
+import { AgentThread } from './AgentThread';
+import { Composer } from './Composer';
+import { IndexingStatus, type IndexingPhase } from './IndexingStatus';
+
+export interface ReedyAssistantProps {
+  appService: AppService;
+  bookDoc: BookDoc;
+  bookHash: string;
+  bookKey: string;
+  aiSettings: AISettings;
+  readingContext: ReadingContextSnapshot;
+  /** Wired by the notebook to `getView(bookKey)?.goTo(cfi)` on click. */
+  onNavigateToCfi?: (cfi: string) => void;
+}
+
+/**
+ * Top-level entrypoint mounted by the notebook AI tab when
+ * `aiSettings.reedy.runtime === 'agent'` (Phase 4.3).
+ *
+ * Constructs ReedyDb / BookIndexer / BookRetriever / tool registry /
+ * AgentRuntime from the per-book deps and renders the indexing status
+ * → AgentThread → Composer flow. The legacy MVP path stays at the
+ * notebook level under the same flag's 'mvp' value.
+ */
+export function ReedyAssistant({
+  appService,
+  bookDoc,
+  bookHash,
+  aiSettings,
+  readingContext,
+  onNavigateToCfi,
+}: ReedyAssistantProps) {
+  const models = useMemo(() => createReedyModels(aiSettings), [aiSettings]);
+
+  // Lazily open reedy.db on first mount. The promise resolves once and
+  // we share the same ReedyDb + Indexer + Retriever for the lifetime of
+  // this component instance.
+  const [reedy, setReedy] = useState<{
+    db: ReedyDb;
+    indexer: BookIndexer;
+    retriever: BookRetriever;
+  } | null>(null);
+  useEffect(() => {
+    let alive = true;
+    void appService
+      .openDatabase('reedy', 'reedy.db', 'Data', { experimental: ['index_method'] })
+      .then((svc) => {
+        if (!alive) return;
+        const db = new ReedyDb(svc);
+        setReedy({ db, indexer: new BookIndexer(db), retriever: new BookRetriever(db) });
+      })
+      .catch((err) => {
+        console.error('[Reedy] failed to open reedy.db', err);
+      });
+    return () => {
+      alive = false;
+    };
+  }, [appService]);
+
+  // Snapshot the reading context in a ref so the tool factories don't
+  // re-render the registry on every page turn.
+  const readingRef = useRef(readingContext);
+  useEffect(() => {
+    readingRef.current = readingContext;
+  }, [readingContext]);
+
+  // Build the tool registry + runtime once per (reedy ready, model) pair.
+  const runtime = useMemo(() => {
+    if (!reedy) return null;
+    const reg = new ToolRegistry();
+    reg.register(createGetReadingContextTool(() => readingRef.current));
+    reg.register(createGetSelectionTool(() => readingRef.current.selection ?? null));
+    reg.register(
+      createLookupPassageTool({
+        bookHash,
+        retriever: reedy.retriever,
+        activeEmbeddingModel: models.embedding,
+      }),
+    );
+    reg.register(
+      createAddCitationTool(() => {
+        // Citation side-channel — the runtime synthesizes a citation
+        // event from the lookupPassage tool result, so addCitation
+        // just acks. A future enhancement could push directly into
+        // the store via a closure here.
+      }),
+    );
+
+    const layers = [
+      createPolicyLayer(DEFAULT_POLICY),
+      createSkillLayer(null),
+      createReadingLayer(readingRef.current),
+      createToolCatalogLayer(reg.list()),
+    ];
+
+    return new AgentRuntime({ model: models.chat, tools: reg, layers });
+  }, [reedy, models.chat, models.embedding, bookHash]);
+
+  const messages = useReedyStore((s) => s.messages);
+  const isRunning = useReedyStore((s) => s.isRunning);
+  const resetStore = useReedyStore((s) => s.reset);
+  const { send, abort } = useReedyTurn(runtime);
+
+  // Indexing state — tracked locally to avoid layering yet another store.
+  const [indexingPhase, setIndexingPhase] = useState<IndexingPhase>('idle');
+  const [indexProgress, setIndexProgress] = useState<{
+    pct: number;
+    current: number;
+    total: number;
+  } | null>(null);
+
+  useEffect(() => {
+    if (!reedy || !bookHash) return;
+    let alive = true;
+    void reedy.db.getBookMeta(bookHash).then((meta) => {
+      if (!alive) return;
+      if (!meta) setIndexingPhase('idle');
+      else if (meta.indexingStatus === 'indexed') setIndexingPhase('indexed');
+      else if (meta.indexingStatus === 'empty_index') setIndexingPhase('empty');
+      else if (meta.indexingStatus === 'failed') setIndexingPhase('failed');
+      else setIndexingPhase('idle');
+    });
+    return () => {
+      alive = false;
+    };
+  }, [reedy, bookHash]);
+
+  // Reset the conversation log when the book or session changes.
+  useEffect(() => {
+    resetStore();
+  }, [bookHash, resetStore]);
+
+  const handleIndex = useCallback(async () => {
+    if (!reedy) return;
+    setIndexingPhase('indexing');
+    try {
+      await reedy.indexer.indexBook(bookDoc, bookHash, models.embedding, {
+        onProgress: (e) => {
+          if (e.phase === 'embedding' && e.total > 0) {
+            setIndexProgress({
+              pct: Math.round((e.current / e.total) * 100),
+              current: e.current,
+              total: e.total,
+            });
+          }
+        },
+      });
+      const meta = await reedy.db.getBookMeta(bookHash);
+      setIndexingPhase(
+        meta?.indexingStatus === 'empty_index'
+          ? 'empty'
+          : meta?.indexingStatus === 'indexed'
+            ? 'indexed'
+            : 'failed',
+      );
+    } catch (err) {
+      console.error('[Reedy] index failed', err);
+      setIndexingPhase('failed');
+    } finally {
+      setIndexProgress(null);
+    }
+  }, [reedy, bookDoc, bookHash, models.embedding]);
+
+  const handleSend = useCallback(
+    (text: string) => {
+      if (!runtime) return;
+      void send({ sessionId: bookHash, bookHash, userMessage: text });
+    },
+    [runtime, send, bookHash],
+  );
+
+  if (!aiSettings.enabled) {
+    return (
+      <div className='flex h-full items-center justify-center p-4'>
+        <p className='text-base-content/60 text-sm'>Enable AI in Settings</p>
+      </div>
+    );
+  }
+
+  if (!reedy) {
+    return (
+      <div className='flex h-full items-center justify-center p-4'>
+        <p className='text-base-content/60 text-sm'>Loading Reedy…</p>
+      </div>
+    );
+  }
+
+  if (indexingPhase !== 'indexed') {
+    return (
+      <IndexingStatus
+        status={indexingPhase}
+        progressPercent={indexProgress?.pct}
+        chunkProgress={
+          indexProgress ? { current: indexProgress.current, total: indexProgress.total } : undefined
+        }
+        onIndex={handleIndex}
+        onReindex={handleIndex}
+      />
+    );
+  }
+
+  return (
+    <div className='reedy-agent-shell flex h-full w-full flex-col'>
+      <div className='min-h-0 flex-1'>
+        <AgentThread
+          messages={messages}
+          isRunning={isRunning}
+          onSourceClick={onNavigateToCfi}
+          emptyState={
+            <div className='text-base-content/60 px-6 text-center text-sm'>
+              Ask anything about this book.
+            </div>
+          }
+        />
+      </div>
+      <Composer isRunning={isRunning} onSend={handleSend} onAbort={abort} />
+    </div>
+  );
+}
diff --git a/apps/readest-app/src/services/reedy/ui/parts/CitationPart.tsx b/apps/readest-app/src/services/reedy/ui/parts/CitationPart.tsx
new file mode 100644
index 00000000..25dd9388
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/parts/CitationPart.tsx
@@ -0,0 +1,45 @@
+'use client';
+
+import { BookOpen } from 'lucide-react';
+import type { ReedyMessagePart } from '../../store/reedyStore';
+
+/**
+ * Citation chip — clickable when an `onClick` handler is wired (the
+ * notebook integration passes a handler that calls
+ * `getView(bookKey)?.goTo(cfi)`). Static when no handler (e.g. preview
+ * rendering in tests).
+ */
+export function CitationPart({
+  part,
+  onClick,
+}: {
+  part: Extract<ReedyMessagePart, { type: 'citation' }>;
+  onClick?: (cfi: string) => void;
+}) {
+  const baseClass =
+    'border-base-content/10 bg-base-200/60 my-1 flex items-start gap-1.5 rounded-md border px-2 py-1.5 text-[11px]';
+  const body = (
+    <>
+      <BookOpen className='text-base-content/60 mt-0.5 size-3 shrink-0' />
+      <div className='flex min-w-0 flex-col gap-0.5 text-start'>
+        <div className='text-base-content font-medium'>
+          {part.chapterTitle ?? `Section ${part.sectionIndex + 1}`}
+        </div>
+        <div className='text-base-content/70 line-clamp-3'>{part.snippet}</div>
+      </div>
+    </>
+  );
+  if (onClick) {
+    return (
+      <button
+        type='button'
+        className={`${baseClass} hover:bg-base-200 transition-colors`}
+        onClick={() => onClick(part.cfi)}
+        title={part.cfi}
+      >
+        {body}
+      </button>
+    );
+  }
+  return <div className={baseClass}>{body}</div>;
+}
diff --git a/apps/readest-app/src/services/reedy/ui/parts/StatusParts.tsx b/apps/readest-app/src/services/reedy/ui/parts/StatusParts.tsx
new file mode 100644
index 00000000..29123741
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/parts/StatusParts.tsx
@@ -0,0 +1,27 @@
+'use client';
+
+import { AlertTriangle, Ban } from 'lucide-react';
+import type { ReedyMessagePart } from '../../store/reedyStore';
+
+export function ErrorPart({ part }: { part: Extract<ReedyMessagePart, { type: 'error' }> }) {
+  return (
+    <div className='border-warning/30 bg-warning/10 my-1 flex items-start gap-1.5 rounded-md border px-2 py-1.5 text-[11px]'>
+      <AlertTriangle className='text-warning mt-0.5 size-3 shrink-0' />
+      <div className='flex min-w-0 flex-col gap-0.5'>
+        <div className='text-warning font-medium'>Error · {part.kind}</div>
+        <div className='text-base-content/70'>{part.message}</div>
+      </div>
+    </div>
+  );
+}
+
+export function AbortPart({ part }: { part: Extract<ReedyMessagePart, { type: 'abort' }> }) {
+  return (
+    <div className='border-base-content/10 bg-base-200/40 my-1 flex items-center gap-1.5 rounded-md border px-2 py-1 text-[11px]'>
+      <Ban className='text-base-content/60 size-3 shrink-0' />
+      <span className='text-base-content/70'>
+        {part.partial ? 'Aborted (partial reply)' : 'Aborted'}
+      </span>
+    </div>
+  );
+}
diff --git a/apps/readest-app/src/services/reedy/ui/parts/TextPart.tsx b/apps/readest-app/src/services/reedy/ui/parts/TextPart.tsx
new file mode 100644
index 00000000..a172baae
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/parts/TextPart.tsx
@@ -0,0 +1,91 @@
+'use client';
+
+import { memo } from 'react';
+import ReactMarkdown from 'react-markdown';
+import remarkGfm from 'remark-gfm';
+
+/**
+ * User-message text rendering. Bare react-markdown with GFM but no
+ * remark-math / rehype-raw / rehype-katex / harden-react-markdown —
+ * the user message is whatever they typed; we don't expect math or HTML
+ * from them. Inline-styled overrides match the legacy MarkdownText
+ * compact look.
+ *
+ * Memoized on `text` so repeated re-renders of the parent thread don't
+ * re-tokenize unchanged user messages.
+ */
+export const UserTextPart = memo(function UserTextPart({ text }: { text: string }) {
+  return (
+    <div className='prose prose-sm dark:prose-invert max-w-none break-words whitespace-pre-wrap'>
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        components={{
+          p: ({ children }) => <span className='inline'>{children}</span>,
+          a: ({ href, children }) => (
+            <a href={href} target='_blank' rel='noopener noreferrer'>
+              {children}
+            </a>
+          ),
+          code: ({ children }) => (
+            <code className='bg-base-300/50 text-base-content rounded px-1.5 py-0.5 font-mono text-sm'>
+              {children}
+            </code>
+          ),
+        }}
+      >
+        {text}
+      </ReactMarkdown>
+    </div>
+  );
+});
+
+/**
+ * Assistant-message text rendering — same react-markdown stack as
+ * UserTextPart, slightly richer block styling (paragraphs render as
+ * `<p>` instead of inline spans). The original plan called for
+ * Streamdown's streaming-aware fade-in spans, but Streamdown statically
+ * depends on Shiki whose TextMate grammars contain lookbehind regex
+ * `(?<=...)` / `(?<!...)` patterns — the repo's `check:lookbehind-regex`
+ * gate rejects them because some Chromium versions on older Android
+ * webviews still don't support lookbehind syntax (see CI run that
+ * landed this fix). Streaming-fade polish returns in a follow-up that
+ * either tree-shakes Shiki out or uses a different streaming renderer.
+ *
+ * Memoized on `text` for the same reason as UserTextPart. The agent
+ * runtime coalesces consecutive text deltas in the store reducer, so
+ * the text grows monotonically per assistant message.
+ */
+export const AssistantTextPart = memo(function AssistantTextPart({ text }: { text: string }) {
+  return (
+    <div className='prose prose-sm dark:prose-invert max-w-none break-words'>
+      <ReactMarkdown
+        remarkPlugins={[remarkGfm]}
+        components={{
+          a: ({ href, children }) => (
+            <a href={href} target='_blank' rel='noopener noreferrer'>
+              {children}
+            </a>
+          ),
+          code: ({ children, className }) => {
+            // Inline code: no className. Fenced blocks: get a language-x class.
+            if (!className) {
+              return (
+                <code className='bg-base-300/50 text-base-content rounded px-1.5 py-0.5 font-mono text-sm'>
+                  {children}
+                </code>
+              );
+            }
+            return <code className={className}>{children}</code>;
+          },
+          pre: ({ children }) => (
+            <pre className='bg-base-300/40 my-2 overflow-auto rounded-md px-3 py-2 font-mono text-xs'>
+              {children}
+            </pre>
+          ),
+        }}
+      >
+        {text}
+      </ReactMarkdown>
+    </div>
+  );
+});
diff --git a/apps/readest-app/src/services/reedy/ui/parts/ToolCallPart.tsx b/apps/readest-app/src/services/reedy/ui/parts/ToolCallPart.tsx
new file mode 100644
index 00000000..f2677290
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/parts/ToolCallPart.tsx
@@ -0,0 +1,88 @@
+'use client';
+
+import { useState } from 'react';
+import {
+  Wrench,
+  ChevronDown,
+  ChevronRight,
+  CheckCircle2,
+  AlertCircle,
+  Loader2,
+} from 'lucide-react';
+import type { ReedyMessagePart } from '../../store/reedyStore';
+
+/**
+ * Renders a tool_call message part as a collapsed pill. Click to expand
+ * the args + result. Pending tools show a spinner; finished tools show
+ * a check (ok) or warning (error) and the duration if available.
+ *
+ * The plan calls for an inline approval UI when permission != 'read';
+ * the runtime's ToolRegistry already enforces approvals via its
+ * requestPermission callback, so that prompt fires at registry-invoke
+ * time. The pill stays informational.
+ */
+export function ToolCallPart({ part }: { part: Extract<ReedyMessagePart, { type: 'tool_call' }> }) {
+  const [expanded, setExpanded] = useState(false);
+  const statusIcon =
+    part.state === 'pending' ? (
+      <Loader2 className='size-3 animate-spin' />
+    ) : part.state === 'error' ? (
+      <AlertCircle className='text-warning size-3' />
+    ) : (
+      <CheckCircle2 className='text-success size-3' />
+    );
+
+  return (
+    <div className='border-base-content/10 bg-base-200/50 my-1 rounded-md border px-2 py-1.5 text-[11px]'>
+      <button
+        type='button'
+        onClick={() => setExpanded((e) => !e)}
+        className='flex w-full items-center gap-1.5 text-start'
+      >
+        {expanded ? <ChevronDown className='size-3' /> : <ChevronRight className='size-3' />}
+        <Wrench className='text-base-content/60 size-3' />
+        <span className='text-base-content font-medium'>{part.name}</span>
+        {statusIcon}
+        {part.durationMs !== undefined && (
+          <span className='text-base-content/40 ms-auto text-[10px]'>{part.durationMs}ms</span>
+        )}
+      </button>
+      {expanded && (
+        <div className='mt-1.5 space-y-1'>
+          <div>
+            <div className='text-base-content/40 mb-0.5 text-[10px] uppercase'>args</div>
+            <pre className='bg-base-300/40 max-h-40 overflow-auto rounded px-1.5 py-1 font-mono text-[10px] whitespace-pre-wrap'>
+              {safeStringify(part.args)}
+            </pre>
+          </div>
+          {part.state === 'ok' && part.result !== undefined && (
+            <div>
+              <div className='text-base-content/40 mb-0.5 text-[10px] uppercase'>result</div>
+              <pre className='bg-base-300/40 max-h-60 overflow-auto rounded px-1.5 py-1 font-mono text-[10px] whitespace-pre-wrap'>
+                {safeStringify(part.result)}
+              </pre>
+            </div>
+          )}
+          {part.state === 'error' && part.error && (
+            <div>
+              <div className='text-error mb-0.5 text-[10px] uppercase'>
+                error · {part.error.kind}
+              </div>
+              <pre className='text-error/80 bg-base-300/40 rounded px-1.5 py-1 font-mono text-[10px] whitespace-pre-wrap'>
+                {part.error.message}
+              </pre>
+            </div>
+          )}
+        </div>
+      )}
+    </div>
+  );
+}
+
+function safeStringify(v: unknown): string {
+  try {
+    return JSON.stringify(v, null, 2);
+  } catch {
+    return String(v);
+  }
+}
diff --git a/apps/readest-app/src/services/reedy/ui/useReedyTurn.ts b/apps/readest-app/src/services/reedy/ui/useReedyTurn.ts
new file mode 100644
index 00000000..e0531f90
--- /dev/null
+++ b/apps/readest-app/src/services/reedy/ui/useReedyTurn.ts
@@ -0,0 +1,71 @@
+'use client';
+
+import { useCallback, useRef } from 'react';
+import type { AgentRuntime, RunTurnInput } from '../runtime/AgentRuntime';
+import type { ReedyEvent } from '../runtime/events';
+import { useReedyStore } from '../store/reedyStore';
+
+/**
+ * Drives one assistant turn through the AgentRuntime, dispatching every
+ * ReedyEvent the runtime yields into the Reedy store (Phase 4.1).
+ *
+ * Why a hook and not a method on the store: the runtime is constructed
+ * by the notebook with per-book deps (model, tools, layers). The store
+ * stays runtime-agnostic; the hook is the glue.
+ */
+export function useReedyTurn(runtime: AgentRuntime | null) {
+  const cancelRef = useRef<AbortController | null>(null);
+
+  const send = useCallback(
+    async (args: { sessionId: string; bookHash: string; userMessage: string }): Promise<void> => {
+      if (!runtime) return;
+      // Cancel any in-flight turn before starting a new one so we never
+      // have two AgentRuntime streams mutating the same active message.
+      cancelRef.current?.abort();
+      const controller = new AbortController();
+      cancelRef.current = controller;
+
+      const store = useReedyStore.getState();
+      store.startUserTurn(args.userMessage);
+
+      const turnInput: RunTurnInput = {
+        sessionId: args.sessionId,
+        bookHash: args.bookHash,
+        userMessage: args.userMessage,
+        signal: controller.signal,
+      };
+
+      // The first event the runtime yields is `turn_start` carrying the
+      // assistantMessageId. Wait for it before pushing the assistant
+      // message into the store so the store has a stable id to mutate.
+      let assistantStarted = false;
+      try {
+        for await (const event of runtime.runTurn(turnInput) as AsyncIterable<ReedyEvent>) {
+          if (!assistantStarted && event.type === 'turn_start') {
+            store.startAssistantTurn(event.assistantMessageId, controller);
+            assistantStarted = true;
+            continue;
+          }
+          if (!assistantStarted) {
+            // Defensive: the runtime contract guarantees turn_start first,
+            // but if something upstream changes don't drop subsequent events.
+            store.startAssistantTurn('msg-fallback', controller);
+            assistantStarted = true;
+          }
+          store.applyEvent(event);
+          if (event.type === 'done') break;
+        }
+      } finally {
+        store.finishTurn();
+        if (cancelRef.current === controller) cancelRef.current = null;
+      }
+    },
+    [runtime],
+  );
+
+  const abort = useCallback(() => {
+    cancelRef.current?.abort();
+  }, []);
+
+  return { send, abort };
+}
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index bb37fdf2..74c68cf5 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -351,6 +351,9 @@ importers:
       react-icons:
         specifier: ^5.4.0
         version: 5.6.0(react@19.2.5)
+      react-markdown:
+        specifier: ^10.1.0
+        version: 10.1.0(@types/react@19.2.14)(react@19.2.5)
       react-responsive:
         specifier: ^10.0.0
         version: 10.0.1(react@19.2.5)
@@ -387,6 +390,9 @@ importers:
       uuid:
         specifier: '>=14.0.0'
         version: 14.0.0
+      virtua:
+        specifier: ^0.49.1
+        version: 0.49.1(react-dom@19.2.5(react@19.2.5))(react@19.2.5)
       ws:
         specifier: 8.20.1
         version: 8.20.1
@@ -8489,6 +8495,26 @@ packages:
     resolution: {integrity: sha512-0QwqXteBNXgnLCdWdvPQBX6FXRHtIH3VhJPTd5Lwn28tJXc34YqSCWUmkOvtJHBmB3gGoPtrOKk3Ts8/kEZ9aA==}
     engines: {node: '>=10.13.0'}
 
+  virtua@0.49.1:
+    resolution: {integrity: sha512-6f79msqg3jzNFdqJiS0FSzhRN1EHlDhR7EvW7emp6z5qQ22VdsReiDHflkpMEMhoAyUuYr69nwT0aagiM7NrUg==}
+    peerDependencies:
+      react: '>=16.14.0'
+      react-dom: '>=16.14.0'
+      solid-js: '>=1.0'
+      svelte: '>=5.0'
+      vue: '>=3.2'
+    peerDependenciesMeta:
+      react:
+        optional: true
+      react-dom:
+        optional: true
+      solid-js:
+        optional: true
+      svelte:
+        optional: true
+      vue:
+        optional: true
+
   vite-plugin-commonjs@0.10.4:
     resolution: {integrity: sha512-eWQuvQKCcx0QYB5e5xfxBNjQKyrjEWZIR9UOkOV6JAgxVhtbZvCOF+FNC2ZijBJ3U3Px04ZMMyyMyFBVWIJ5+g==}
 
@@ -17801,6 +17827,11 @@ snapshots:
       - bare-abort-controller
       - react-native-b4a
 
+  virtua@0.49.1(react-dom@19.2.5(react@19.2.5))(react@19.2.5):
+    optionalDependencies:
+      react: 19.2.5
+      react-dom: 19.2.5(react@19.2.5)
+
   vite-plugin-commonjs@0.10.4:
     dependencies:
       acorn: 8.16.0